would you say the people in car or would
you say the people in the car?

Unfortunately, the english language isn't that consistent. A car is an object,
Usenet isn't (that probably isn't the technically correct reason, but I'm no
professor of english). Let me put it this way, would you say: "people in
Birmingham" or "people in the Birmingham". To me, the latter would only make
sense if there's a river called the Birmingham river and you were referring
to them, rather than people in the city of Birmingham.

Unfortunately, the English language isn't even that consistent. Usenet
is a medium, but we're not even consistent about how we refer to media.
You watch TV, but you listen to the radio and you go to the movies; you
surf the Internet, but you post to Usenet.

I can imagine that in its early days some people may have referred to it
as "the Usenet," but by now usage has settled on simply "Usenet." I
don't think I've ever heard or seen anyone refer to "the Usenet."
(Except maybe in old RFCs?) My two cents.